Cavern tokenisation oddity

[Olly Betts]

I wonder if I should adjust how cavern tokenises. Currently a token is defined as a sequence of consecutive letters, so this is accepted:

*fix2 reference0 0 0

We don't document this as working, and I doubt anyone is deliberately using it, but it's possible there are a few accidental uses. If we're going to change this we probably ought to make it a warning and give people a chance to fix any real world instances.

Seen in current git master, but I believe it's been like this forever.